The present invention relates generally to laser systems. More specifically, it relates to a novel class of tunable lasers that can provide effectively continuous tuning in a tuning range spanning multiple gain spectra in a simple, versatile, and economical way. Such novel tunable lasers are particularly suited for fiber-optic networks and telecommunication component testing applications.
As fiber-optic networks employing wavelength division multiplexing (WDM) become increasingly pervasive as the backbone of modern communications systems, there is a growing demand for tunable laser sources that can provide a wide range of wavelengths in a simple, versatile, and economical way. Such tunable laser sources are desired, for instance, in swept-wavelength testing of passive and active telecommunication components. Tunable laser sources are also employed in multi-channel coherent communication systems, spectroscopic measurements, and optical amplifier characterizations.
Extended (or external) cavity diode lasers (ECDLs) are conventionally employed in the art to provide tunable laser sources for swept-wavelength testing in telecommunications and other applications. For purpose of elucidating the principle and the distinct features of the present invention, the underlying principle of operation of ECDLs is briefly described below. A more detailed description of external cavities is well documented in the art, for example, in xe2x80x9cSpectrally Narrow Pulsed Dye Laser without Beam Expanderxe2x80x9d by Littman et al., Applied Optics, vol.17, no.14, pp.2224-2227, Jul. 15 1978; xe2x80x9cNovel geometry for single-mode scanning of tunable lasersxe2x80x9d by Littman et al., Optics Letters, vol.6, no.3, pp.117-118; xe2x80x9cExternal-Cavity diode laser using a grazing-incidence diffraction gratingxe2x80x9d by Harvey et al., Optics Letters, vol.16, no.12, pp.910-912; and xe2x80x9cWidely Tunable External Cavity Diode Lasersxe2x80x9d by Day et al., SPIE, Vol. 2378, pp.35-41.
In a tunable ECDL, as the name suggests, the wavelength selection and tuning functions are external to the gain element where the laser action takes place. Such a system typically utilizes an external cavity of variable length in conjunction with a diffraction grating and a movable mirror (or simply-a movable diffraction grating), all external to a semiconductor diode (serving as a gain element). An incident laser beam is diffracted by the grating. A diffracted beam with the desired wavelength is selected by the movable mirror, further reflected back onto the diffraction grating, and subsequently transmitted back to the semiconductor diode where further amplification takes place. Rotation and/or translation of the movable mirror enables the system to be tuned to different wavelengths. (Alternatively, the movable diffraction grating is rotated/translated, to provide tunability in wavelength.) The ultimate limit to the tuning range is set by the gain spectrum of the semiconductor diode.
In an ECDL, the number of nodal points of the standing wave in the laser cavity is proportional to L/xcex, where xcex is the operating wavelength and L is the total optical length of the laser cavity (primarily provided by the length Lext of the external cavity). Therefore, if the wavelength tuning takes place while L is maintained constant, the number of nodal points in the laser cavity changes discontinuously. That is, the wavelength cannot be continuously varied, but rather, it leaps in discrete stepsxe2x80x94termed as mode-hops. As a result, it is often difficult to tune in a desired wavelength, and there may also be substantial fluctuations in the output power of the laser. Mode-hops can be avoided by varying the length L of the laser cavity as the wavelength tuning takes place (such that as the tuning passband of the diffraction grating shifts in response to the tuning, the underlying axial modes of the laser cavity follow accordingly), in a coordination that requires great accuracy and stringent tolerance. Coordinating the wavelength tuning and the cavity-length changing in ECDLs has been a rather arduous and expensive undertaking.
Efforts have been made in the art to preventing mode-hops and thereby providing more continuous tuning, as exemplified by U.S. Pat. Nos. 5,172,390, 5,319,668, 5,347,527, 5,491,714, 5,493,575, 5,594,744, 5,862,162, 5,867,512, 6,026,100, 6,038,239, 6,115,401, and 6,134,250. For example, U.S. Pat. No. 5,319,668 describes an external cavity semiconductor laser, comprising a semiconductor laser diode, a diffraction grating and a movable mirror. The movable mirror is mounted on a pivot so positioned to provide simultaneous rotation and linear translation, thereby enabling continuous single-mode tuning. The mirror pivot point is determined by a detailed calculation which takes into account a number of factors in the laser cavity, so as to maintain a precise control of the length of the laser cavity. U.S. Pat. No. 5,347,527 discloses a tunable external cavity laser source and a process for adjusting the laser source, such that continuous tunability can be provided. U.S. Pat. No. 5,867,512 describes an external cavity semiconductor laser and an elaborate tuning arrangement for avoiding mode-hops. Particular effort is made in this patent to correct the chromatic dispersion effects in the laser cavity. The appearance of these prior art patents (along with many others) in fact serve as a testimony of the difficulty with combating mode-hops in ECDLs.
U.S. Pat. No. 6,115,401 discloses a laser system, in which a gain medium (such as a semiconductor diode) is optically coupled to an external cavity containing a monolithic prism assembly. The monolithic prism assembly, including a transparent substrate carrying a Fabry-Perot thin film interference filter, plays the role of the diffraction grating in a conventional ECDL (as described above). Translation of the monolithic prism assembly provides continuous mode-hop-free tuning of the laser operating wavelength. The intent of this invention is to make a compact single-frequency tunable laser with very narrow linewidths, primarily aiming at dense wavelength division multiplexing (DWDM) applications.
U.S. Pat. No. 6,134,250 describes a single-mode wavelength selectable ring laser, which operates at a single wavelength selectable from any channel passband of a multiple-channel wavelength multiplex/demultiplex element (e.g., an arrayed waveguide grating router (AWGR)). A Fabry-Perot semiconductor optical amplifier (FP-SOA) is connected to AWGR to form a ring laser structure, where FP-SOA is used as an intra-cavity narrow-band mode-selecting filter to stabilize the laser oscillation to a single axial mode. As such, this ring laser system can only provide discrete tuning from one wavelength passband of the wavelength filter to another. That is, continuous tuning cannot be achieved in this system. It should be noted that although this prior art patent discloses a configuration in which several semiconductor optical amplifiers (SOAs) are implemented in a demultiplexer AWGR, each of the SOAs is dedicated to amplify an optical signal with a particular wavelength. That is, these SOAs function merely as wavelength switches. And more important is the fact that the overall tuning range of this laser system is limited by the single gain spectrum of FP-SOA. Hence, this prior art laser system is suited as providing a wavelength-selectable laser, as opposed to a tunable laser.
Daneu et al. in xe2x80x9cSpectral beam combining of a broadstrip diode laser in an external cavityxe2x80x9d, CLEO 2000, describes a xe2x80x9cspectral beam combiningxe2x80x9d laser system, in which several diode lasers placed in parallel are being used simultaneously, each having the identical gain spectrum but lasing at a different wavelength. By having several lasers operating simultaneously, the collective output beam of the system has many wavelengths superimposed, thereby providing a higher power. However, the overall tuning range of this system is no more than what a single diode is able to provide. Moreover, the external cavity in this case does not employ an optical fiber.
Despite various efforts that have been undertaken, the prior art ECDLs still suffer a number of shortcomings, summarized as follows:
a) Since only one gain element (e.g., a semiconductor laser diode) is employed, or only one gain spectrum is effectively in action, the overall tuning range in the prior art ECDLs is rather limited. As a way of example, these ECDLs typically have tuning ranges of 50-100 nm in wavelength (centered about 1550 nm for telecommunications applications). A wider tuning range is in demand for many applications. For instance, the next-generation telecommunications components would require testing over a tuning range of at least 250 nm.
b) In the prior art ECDLs intended to provide continuous tuning, the axial-mode spacing is typically on the order of several GHz, which imposes a tuning resolution that is too large for many applications (such as swept-wavelength testing of telecommunication components) to tolerate. In order to prevent mode-hops and thereby provide more continuous tuning, an elaborate mechanism must be implemented in an ECDL, which requires stringent mechanical tolerance and painstaking adjustment. Such stringent tolerance and intricate alignment further render the ECDL thus constructed highly vulnerable to even minor errors in adjustment.
c) The prior art ECDLs typically have very narrow linewidths (e.g., less than 1 MHz), which not only are unnecessary in many practical applications, but also inadvertently introduce adverse effects. For instance, in applications where light is launched into an optical fiber and propagates over a long distance at low power, or over a short distance at high power, the presence of narrow linewidths along with either of these conditions renders the system susceptible to unwanted stimulated brilliuon scattering (SBS). Narrow linewidths may also be a disadvantage in a complex system, where there are lengthy etalons disposed between various components and/or interfaces.
These shortcomings render the prior art ECDLs difficult and expensive to build and implement in practice, and impede their wider applications.
In view of the foregoing, there is a need for a tunable laser source that overcomes the prior art problems in a simple and low cost construction.
The aforementioned need in the art is provided by a tunable laser of the present invention, comprising a plurality of gain elements having a plurality of gain spectra, a splitting-combining means, and a wavelength-selecting means. The gain elements are optically coupled to the splitting-combining means in parallel, and the splitting-combining means is in optical communication with the wavelength-selecting means. The gain spectra are mutually distinct in general, so as to provide a wider tuning range. The tunable laser of the present invention further comprises an axial-mode control means, for determining the axial-mode structure of the laser cavity.
In this specification and appending claims, two gain spectra are said to be xe2x80x9cdistinctxe2x80x9d, if there is at least one wavelength at which one of the two gain spectra displays practically significant (positive) gain while the other one does not. Two distinct gain spectra can be xe2x80x9cpartially overlappingxe2x80x9d in wavelength: that is, there are one or more wavelengths at which both of the gain spectra displaying practically significant (positive) gain. Two distinct gain spectra can alternatively be xe2x80x9csubstantially non-overlappingxe2x80x9d: namely, there is no common wavelength at which both of the two gain spectra display positive gain. Moreover, a plurality of gain spectra are said to be xe2x80x9cmutually distinctxe2x80x9d, if each member of the gain spectra is xe2x80x9cdistinctxe2x80x9d with respect to all other members of the gain spectra. This can be provided, for instance, by a plurality of gain spectra that are partially overlapping in a successive and incremental manner along wavelength. It can also be provided by a plurality of gain elements that are substantially non-overlapping in wavelength, thereby providing a plurality of discrete gain spectra. It can be further provided by a plurality of gain spectra in which some members of the gain spectra are partially overlapping, while others are substantially non-overlapping in wavelength.
In the present invention, the gain elements can be selected from the group of solid state gain media such as semiconductor diodes, doped fibers, doped crystals, doped glasses, and the like known in the art. The wavelength-selecting means can be provided by a diffraction grating, a prism, an acousto-optic filter, a tunable interference filter, a tunable birefrigent filter, a tunable etalon, or other wavelength-selecting elements known in the art. The splitting-combining means can be a fiber-optic coupler, a wavelength-division-multiplexing (WDM) coupler, a wavelength router, an active switching means, or a combination of these elements.
In general, the splitting-combining means should be configured such that there are N (Nxe2x89xa72) I-ports and M (Mxe2x89xa71) O-ports. The N I-ports are connected to N gain elements in a one-to-one correspondence. One of the O-ports is connected to the wavelength-selecting means, and any of the remaining (Mxe2x88x921) O-ports may be utilized to provide one or more output ports for the laser system. The splitting-combining means serves two functions: 1) at a given time, it routes/splits a back-coupled beam transmitted from the wavelength-selecting means along one or more paths and passes one or more sub-beams thus obtained into one or more gain elements; and 2) it combines the light amplified by one or more gain elements into a forward-coupled beam and directs the forward-coupled beam to the wavelength-selecting means. Furthermore, in the course of wavelength tuning, the splitting-combining means splits/routes the back-coupled beams along two or more paths into two or more gain elements. For instance, if an Nxc3x97M fiber-optic coupler is used to provide a splitting-combining means, a back-coupled beam is (nearly) uniformly split along N paths, and the N sub-beams thus created are directed to all N gain elements. By contrast, if a wavelength-dependent element such as a wavelength router is used to serve as a splitting-combing means, a back-coupled beam with the selected wavelength at a given time may be routed only to a specific gain element which is capable of providing further amplification at the selected wavelength.
In the present invention, the axial-mode control means may be provided by an optical fiber interposed between, and in optical communication with, the splitting-combining means and the wavelength-selecting means, thereby serving as a substantial portion of an external cavity. The length of the optical fiber can be used to control the axial-mode structure of the laser cavity, for the spacing between two adjacent axial modes is given by c/2L, where c is the speed of light and L is the total optical length of the laser cavity. The length of the optical fiber can be further varied by coupling a piezoelectric fiber stretcher to the fiber, as a way of example. The axial-mode control means can alternatively (or additionally) be provided by a dispersive means, comprising one or more elements selected from the group of prisms, diffraction gratings, high-dispersion fibers, dispersive materials, and the like known in the art. The axial-mode control means can be further provided by an active-modulation means, including one or more elements selected from the group of pump-source-gain-modulation elements, piezoelectric modulators, electro-optic modulators, and acousto-optic modulators, and other modulation elements known in the art.
The tunable laser of the present invention operates as follows. A forward-coupled laser beam is transmitted from the gain elements (e.g., N semiconductor diodes) to the wavelength-selecting means (e.g, a diffraction grating optically coupled to a movable mirror) by way of the splitting-combining beams (e.g., an Nxc3x97M fiber-optic coupler). As a way of example to illustrate the general principle of the present invention, the gain elements are characterized by a plurality of gain spectra that are mutually distinct (e.g., the gain spectra are partially overlapping in a successive manner along wavelength); and the axial-mode control means is provided by a long optical fiber optically coupling the splitting-combining means to the wavelength-selecting means. The wavelength-selecting means in turn transmits a back-coupled beam with the selected wavelength back to the splitting-combining means via the optical fiber. The splitting-combining means further directs the back-coupled beam into one or more gain elements. Further amplification takes place in one or more gain elements whose gain spectra provide significant (positive) gain at the selected wavelength. Subsequent rotation and/or translation of the movable mirror selects a different wavelength, which is further amplified by one or more different gain elements whose gain spectra show significant gain at the subsequent-selected wavelength. Hence, as the wavelength is tuned by way of the wavelength-selecting means, the constituent gain elements with distinct gain spectra take turns to amplify the successively selected wavelengths. As such, the availability of a plurality of gain elements and hence a plurality of distinct gain spectra greatly enhances the tuning range of the laser system of the present invention.
Moreover, by employing a long optical fiber to provide a substantial portion of the external cavity in a tunable laser of the present invention, the resulting axial-mode spacing is so small that mode-hops become inconsequential for practical purposes. This not only enables the wavelength tuning to be effectively continuous, but also greatly simplifies the construction and thereby lowers the cost of the tunable laser of the present invention.
Additionally, by choosing the gain elements with appropriate gain spectra and/or utilizing the gain spectra according to predetermined schemes in the course of wavelength tuning, the tunable laser of the present invention attains a versatile tunability that is unprecedented in the prior art systems. For example, by utilizing a plurality of gain spectra that are partially overlapping in a successive and incremental manner along wavelength, the tunable laser of the present invention provides a continuous-tuning laser source with a tuning range that can be arbitrarily chosen. Alternatively, by using a plurality of gain spectra that are substantially non-overlapping in wavelength, the tunable laser of the present invention effectively acts as a wavelength-selectable laser source, which is capable of providing a wide selection of output wavelengths.
Moreover, the gain elements can be divided into groups according to their gain spectra, and different groups are then alternately turned off while the wavelength tuning takes place. This way of operation ensures that only one gain element is in action during operation, thereby preventing the cross-over noise (such as mode-beating) that tends to arise when two gain elements are simultaneously in action. It may be further used to provide alternate collections of output wavelengths. All in all, the availability of a plurality of gain elements and hence a plurality of distinct gain spectra provides addition avenues for wavelength selecting/tuning, thereby imparting greater utility to the tunable laser of the present invention.
As such, the tunable laser of the present invention provides many advantages over the prior art tunable laser systems, summarized as follows:
By advantageously employing a plurality of gain elements with a plurality of distinct gain spectra, the tunable laser of the present invention provides a wide tuning range that is effectively all-wavelength available. As a way of example, a tuning range of at least 200 nm can be achieved in a tunable laser of the present invention.
By utilizing a long optical fiber as a substantial portion of an external cavity, the axial-mode spacing becomes so small that mode-hops can be effectively ignored in a tunable laser of the present invention. This removes the need for an intricate construction with stringent tolerance that is prevalent in the prior art ECDLS, thereby rendering the tunable laser of the present invention a simpler and lower cost construction.
By advantageously exploiting the availability of a plurality of distinct gain spectra according to various predetermined schemes, the tunable laser of the present invention can be configured as a continuous-tuning laser source, a wavelength-selectable laser source, or a switchable laser source providing various collections of desirable wavelengths.
All in all, the present invention provides a versatile and robust tunable laser in a simple and low cost construction.